Electron Microbunches Research Articles

Quasi-monoenergetic ultrashort microbunch electron source

By combining static electric and laser fields for generation of field-emitted electrons, it is possible to generate a quasi-monoenergetic train of electron microbunches by controlling the anode–cathode spacing such that the time of flight of the electrons becomes independent of the laser field. Such quasi-monoenergetic microbunches with pulse durations that are a fraction of the laser wavelength would be ideal for radiation sources as well as compact accelerators.

Magnetic design of an Apple-X afterburner for the SASE3 undulator of the European XFEL

In its startup configuration the SASE3 beamline of the European XFEL provides only soft X-ray radiation, linearly polarized in the horizontal plane. In order to enhance capabilities of this beamline an afterburner scheme is proposed. It will be used as a coherent radiator using the micro-bunched electron beam of the linear SASE3 system. Radiation with variable polarization, which covers the full SASE3 wavelength range can be generated. For the radiator a new type of undulator design called Apple-X will be used. In this paper the design is described and magnet parameters, which are compatible with the SASE3 afterburner are determined using RADIA simulations. The end structure of such a device is optimized for minimum 1st field integrals.

Brilliant X-rays using a Two-Stage Plasma Insertion Device

Particle accelerators have made an enormous impact in all fields of natural sciences, from elementary particle physics, to the imaging of proteins and the development of new pharmaceuticals. Modern light sources have advanced many fields by providing extraordinarily bright, short X-ray pulses. Here we present a novel numerical study, demonstrating that existing third generation light sources can significantly enhance the brightness and photon energy of their X-ray pulses by undulating their beams within plasma wakefields. This study shows that a three order of magnitude increase in X-ray brightness and over an order of magnitude increase in X-ray photon energy is achieved by passing a 3 GeV electron beam through a two-stage plasma insertion device. The production mechanism micro-bunches the electron beam and ensures the pulses are radially polarised on creation. We also demonstrate that the micro-bunched electron beam is itself an effective wakefield driver that can potentially accelerate a witness electron beam up to 6 GeV.

Status of the development of Delhi Light Source (DLS) at IUAC

A project to construct a compact pre-bunched Free Electron Laser by using a normal conducting photocathode electron gun has been undertaken at Inter University Accelerator Centre (IUAC), New Delhi, India. In this facility, the short laser pulses from a high power laser system will be split into many pulses (2–16) commonly known as ‘Comb beam’ and will strike the photocathode material (metal and semiconductor) to produce electron beam bunches. The electrons will be accelerated up to an energy of ∼8MeV by a copper cavity operated at a frequency of 2860MHz and the beam will be injected into a compact, planar permanent undulator magnet to produce THz radiation. The radiation frequency designed to be tuned in the range of 0.15–3THz by varying the magnetic field of the undulator and/or changing the energy of the electron. The separation of the laser micro-pulses will be varied by adjusting the path length difference to alter the separation of the electron micro-bunches and to maximise the radiation intensity.

Generation of Intense Narrow-Band Tunable Terahertz Radiation from Highly Bunched Electron Pulse Train

We present the analysis and start-to-end simulation of an intense narrow-band terahertz (THz) source with a broad tuning range of radiation frequency, using a single-pass free electron laser (FEL) driven by a THz-pulse-train photoinjector. The fundamental radiation frequency, corresponding to the spacing between the electron microbunches, can be easily tuned by varying the spacing time between the laser micropulses. Since the prebunched electron beam is highly bunched at the first several harmonics, with the harmonic generation technique, the radiation frequency range can be further enlarged by several times. The start-to-end simulation results show that this FEL is capable of generating a few tens megawatts power, several tens micro-joules pulse energy, and a few percent bandwidth at the frequencies of 0.5–5 THz. In addition, several practical issues are considered.

High efficiency, multiterawatt x-ray free electron lasers

We study high efficiency, multi-terawatt peak power, few angstrom wavelength, X-ray Free Electron Lasers (X-ray FELs). To obtain these characteristics we consider an optimized undulator design: superconducting, helical, with short period and built-in strong focusing. This design reduces the length of the breaks between modules, decreasing diffraction effects, and allows using a stronger transverse electron focusing. Both effects reduce the gain length and the overall undulator length. The peak power and efficiency depend on the transverse electron beam distribution and on time dependent effects, like synchrotron sideband growth. The last effect is identified as the main cause for reduction of electron beam microbunching and FEL peak power. We show that the optimal functional form for the undulator magnetic field tapering profile, yielding the maximum output power, depends significantly on these effects. The output power achieved when neglecting time dependent effects for an LCLS-like X-ray FEL with a 100 m long tapered undulator is 7.3 TW, a 14 $\%$ electron beam energy extraction efficiency. When these effects are included the highest peak power is achieved reducing the tapering rate, thus minimizing the reduction in electron micro-bunching due to synchrotron sideband growth. The maximum efficiency obtained for this case is 9 $\%$, corresponding to 4.7 TW peak radiation power. Possible methods to suppress the synchrotron sidebands, and further enhance the FEL peak power, up to about 6 TW by increasing the seed power, are discussed.

Measurements and simulations of seeded electron microbunches with collective effects

Measurements of the longitudinal phase-space distributions of electron bunches seeded with an external laser were done in order to study the impact of collective effects on seeded microbunches in free-electron lasers. When the collective effects of Coulomb forces in a drift space and coherent synchrotron radiation in a chicane are considered, velocity bunching of a seeded microbunch appears to be a viable alternative to compression with a magnetic chicane under high-gain harmonic generation seeding conditions. Measurements of these effects on seeded electron microbunches were performed with a rf deflecting structure and a dipole magnet which streak out the electron bunch for single-shot images of the longitudinal phase-space distribution. Particle tracking simulations in 3D predicted the compression dynamics of the seeded microbunches with collective effects.

Femtosecond visualization of laser-induced optical relativistic electron microbunches.

It has long been known that lasers can interact with relativistic electrons in magnetic undulators to imprint sinusoidal modulations that can be used to slice electrons into microbunches equally separated at the laser wavelength. Here we report on the first direct measurement of laser-induced microbunching of a relativistic electron beam with femtosecond resolution in the time domain. Using a modified zero-phasing technique to map the electron beam's temporal structures into the energy space, we show that this method can be used to directly quantify the time and spectral content of coherent current modulations imprinted on the beam for harmonic and multicolor lasing applications in accelerator-based light sources.

Vacuum electron acceleration by using two variable frequency laser pulses

A method is proposed for producing a relativistic electron bunch in vacuum via direct acceleration by using two frequency-chirped laser pulses. We consider the linearly polarized frequency-chiped Hermit-Gaussian (0, 0) mode lasers with linear chirp in which the local frequency varies linearly in time and space. Electron motion is investigated through a numerical simulation using a three-dimensional particle trajectory code in which the relativistic Newton's equations of motion with corresponding Lorentz force are solved. Two oblique laser pulses with proper chirp parameters and propagation angles are used for the electron acceleration along the z-axis. In this way, an electron initially at rest located at the origin could achieve high energy, γ=319 with the scattering angle of 1.02∘ with respect to the z-axis. Moreover, the acceleration of an electron in different initial positions on each coordinate axis is investigated. It was found that this mechanism has the capability of producing high energy electron microbunches with low scattering angles. The energy gain of an electron initially located at some regions on each axis could be greatly enhanced compared to the single pulse acceleration. Furthermore, the scattering angle will be lowered compared to the acceleration by using laser pulses propagating along the z-axis.

Microbunched Electron Cooling for High-Energy Hadron Beams

Electron and stochastic cooling are proven methods for cooling low-energy hadron beams, but at present there is no way of cooling hadrons as they near the TeV scale. In the 1980s, Derbenev suggested that electron instabilities, such as free-electron lasers, could create collective space charge fields strong enough to correct the hadron energies. This Letter presents a variation on Derbenev's electron cooling scheme using the microbunching instability as the amplifier. The large bandwidth of the instability allows for faster cooling of high-density beams. A simple analytical model illustrates the cooling mechanism, and simulations show cooling rates for realistic parameters of the Large Hadron Collider.

High-Gain Thompson-Scattering X-Ray Free-Electron Laser by Time-Synchronic Laterally Tilted Optical Wave

A novel approach to generating coherent x rays with 10(9)-10(10) photons and femtoseconds duration per laser pulse is proposed. This high intensity x-ray source is realized first by the pulse front tilt of a lateral fed laser to extend the electron-laser synchronic interaction time by several orders, which accomplishes the high-gain free-electron-laser-type exponential growth process and coherent emission with highly microbunched electron beam. Second, two methods are presented to enhance the effective optical undulator strength parameter. One is to invoke lenses to focus two counterpropagating lasers that are at normal incidence to the electron beam as a transverse standing wave; the other is to invent a periodic microstructure that can significantly enhance the center electromagnetic field realized by a resonant standing wave and the quadrupole waveguides. The energy coupling efficiency between the electron beam and laser is therefore greatly improved to generate the high brightness x rays, which is demonstrated by analytical and simulation results.

Echo enabled harmonic generation free electron laser in a mode-locked configuration

Echo Enabled Harmonic Generation (EEHG) is a method of harmonic up-shifting proposed to extend the temporal coherence properties of Free Electron Lasers (FEL) at shorter wavelengths where coherent laser seed fields are not available. Previous theoretical studies of EEHG have applied periodic boundary conditions to the electron distribution in phase space. It is shown that when these periodic boundary conditions are removed a temporal comb of enhanced electron microbunching is revealed. By matching this comb structure in the electron microbunching to the radiation modes that are generated in a Mode-Locked Optical Klystron (MLOK) FEL configuration, a train of short radiation pulses can be generated.

Development of compact coherent EUV source based on laser Compton scattering

High-power extreme ultra-violet (EUV) sources are required for next generation semiconductor lithography. We start to develop a compact EUV source in the spectral range 13–14 nm, which is based on a laser Compton scattering between a 7 MeV micro-bucnhed electron beam and a high-intensity CO 2 laser pulse. The electron beam extracted from a DC photocathode gun is micro-bunched using a laser modulation techinque with the Compton wavelength at a harmonic of the seeding laser before the main laser Compton scattering for EUV generation. A considerating scheme for the compact EUV source based on the laser Compton scattering with micro-bunched electron beam and the analytical study of micro-bunch generation are described in this paper.

PASER – particle acceleration by stimulated emission of radiation: theory, experiment, and future applications

AbstractWe review the theory and the experimental demonstration of one of the novel advanced particle acceleration techniques dubbed PASER ‐ for particle acceleration by stimulated emission of radiation. In such an acceleration paradigm, bundles of electrons are accelerated using the same principle as in laser. A proof‐of‐principle experiment, conducted at the Brookhaven National Laboratory, demonstrated this novel acceleration scheme. A 45 MeV electron macrobunch was modulated by a high‐power CO2 laser in a wiggler, and then injected into an excited CO2 gas mixture. The emerging microbunches reveal a 0.15 % relative change in the kinetic energy in less than 40 cm long interaction region. This is the first experimental demonstration of coherent collisions of the second kind. The fundamental physics underlying the PASER paradigm is discussed via the analysis of a two‐dimensional analytic model used to evaluate the energy exchange occurring as a train of electron microbunches traverses an active medium. Furthermore, advanced PASER concepts such as non‐linear interaction of electrons and waves in an active medium, the occurrence of resonant absorption instability, and PASER‐based optical Bragg accelerators, are considered as well. The possible impact of PASER‐based accelerators on future compact medical accelerators, and on high‐energy physics applications is discussed.

Chaoticity parameter λ and multiple coherent components in relativistic heavy-ion collisions

Three-pion correlations are conventionally analyzed using a chaotic component and a single coherent component of the emitted pions. The existence of source coherence in these experiments is suggested, but the detailed structure of the coherent component remains obscure. An improved understanding of the coherent processes is obtained by incorporating into the model general properties of pulsed radiation, inspired by the free-electron laser operation with multiple, mutually independent coherent source currents due to electron microbunching. This approach with multiple coherent components solves an inconsistency in the earlier three-pion data analysis and leads to a more appealing description of the experimental results.

Enhanced high-gain harmonic generation for x-ray free-electron laser

The harmonic generation free-electron laser (HG-FEL) is capable of producing temporally and spatially coherent pulses in the vacuum ultraviolet and shorter wavelength regions. In this letter an easy-to-implement scheme that significantly enhances the performance of HG-FEL and extends its short-wavelength range is proposed. By adding a short phase-reversed modulator after the dispersive section, the electron energy spread is reduced and simultaneously the electron microbunching is enhanced. Three-dimensional simulations demonstrate the energy spread reduction and the bunching enhancement and show that more powerful higher harmonic radiation can be obtained.

Generation of Trains of Electron Microbunches with Adjustable Subpicosecond Spacing

We demonstrate that trains of subpicosecond electron microbunches, with subpicosecond spacing, can be produced by placing a mask in a region of the beam line where the beam transverse size is dominated by the correlated energy spread. We show that the number, length, and spacing of the microbunches can be controlled through the parameters of the beam and the mask. Such microbunch trains can be further compressed and accelerated and have applications to free electron lasers and plasma wakefield accelerators.

Production and characterization of attosecond electron bunch trains

We report the production of optically spaced attosecond electron microbunches produced by the inverse free-electron-laser (IFEL) process. The IFEL is driven by a Ti:sapphire laser synchronized with the electron beam. The IFEL is followed by a magnetic chicane that converts the energy modulation into the longitudinal microbunch structure. The microbunch train is characterized by observing coherent optical transition radiation (COTR) at multiple harmonics of the bunching. Experimental results are compared with 1D analytic theory showing good agreement. Estimates of the bunching factors are given and correspond to a microbunch length of 410 attosec FWHM. The formation of stable attosecond electron pulse trains marks an important step towards direct laser acceleration.

Seeded free-electron and inverse free-electron laser techniques for radiation amplification and electron microbunching in the terahertz range

A comprehensive analysis is presented that describes amplification of a seed THz pulse in a single-pass free-electron laser (FEL) driven by a photoinjector. The dynamics of the radiation pulse and the modulated electron beam are modeled using the time-dependent FEL code, GENESIS 1.3. A 10-ps (FWHM) electron beam with a peak current of 50 ‐100 A allows amplification of a � 1k Wseed pulse in the frequency range 0.5‐3 THz up to 10 ‐100 MW power in a relatively compact 2-m long planar undulator. The electron beam driving the FEL is strongly modulated, with some inhomogeneity due to the slippage effect. It is shown that THz microbunching of the electron beam is homogeneous over the entire electron pulse when saturated FEL amplification is utilized at the very entrance of an undulator. This requires seeding of a 30cm long undulator buncher with a 1‐3 MW of pump power with radiation at the resonant frequency. A narrow-band seed pulse in the THz range needed for these experiments can be generated by frequency mixing of CO2 laser lines in a GaAs nonlinear crystal. Two schemes for producing MW power pulses in seeded FELs are considered in some detail for the beam parameters achievable at the Neptune Laboratory at UCLA: the first uses a waveguide to transport radiation in the 0.5‐3 THz range through a 2-m long FEL amplifier and the second employs high-gain third harmonic generation using the FEL process at 3‐9 THz.

Particle acceleration by stimulated emission of radiation: Theory and experiment

The interaction of electromagnetic radiation with free electrons in the presence of an active medium has some appealing outcomes. Among them is particle acceleration by stimulated emission of radiation (PASER). In its framework, energy stored in an active medium (microscopic cavities) is transferred directly to an e -beam passing through. We have developed a two-dimensional analytic model for the evaluation of the energy exchange occurring as a train of electron microbunches traverses a dilute resonant medium. Efficient interaction occurs at resonance-namely, when the frequency of the train matches the resonance frequency of the medium. It is shown that the energy exchange is gamma independent for relativistic energies and it drops dramatically with an increase of the beam's radius. Based on this model, we have evaluated the relative change in the kinetic energy of a 0.1-nC 45-MeV macrobunch traversing an excited CO2 gas mixture-the former being modulated at the CO2 laser wavelength. Good agreement is found between the theoretical predictions and the results of the PASER experiment performed recently at Brookhaven National Laboratory.